The aims of this research are: 1) To investigate by gel electrophoresis a number of genetic loci in populations of closely related species of Drosophila for which a cytological phylogeny is available. We wish to determine what fraction of the genome displays genetic variability between populations of the same or different rank, the amount of genetic difference between populations, species, phylads and so on, and to relate this information to the evolutionary history of the group in such a way as to document the changes undergone by gene pools during speciation, during ecological shifts, during intercontinental colonizing episodes, and so on. Among other things, this provides us with direct evidence as to differential rates of evolution for different loci, for different loci in different lineages, and for the different lineages themselves. In special cases we are analyzing large population samples to document intraspecific differentiation and relating this to the amount and nature of the reproductive isolation that exists between them. 2) In appropriate cases we are further investigating genetic variability as this is evidenced by the application of additional criteria such as heat stability, pH sensitivity, etc. By the application of heat sensitivity we have discovered that electrophoresis underestimates the number of alleles at different loci by at least a factor of two. In addition we are studying the biochemical and physiological properties of the different allozymes by various kinetic studies.